Background: I have no background in medical sciences, and this is something I've been thinking about since the start of the Covid-19 pandemic; please ELI5 your answer if you can. Thanks!

We estimate the age of the Earth to be in the hundreds of millions of years old, and the age of human society to be in the tens of thousands of years. One would therefore posit, modulo changes in environment, that pretty much every naturally occurring virus that exists, has probably been discovered, or at the very least, has been experienced and has some kind of history; even if we didn't know what it was at the time, we could at least look back at historical records and go "oh, that was that disease, now we know what it is and can treat it".

When the Covid-19 pandemic began, scientists were not sure if it was natural or made in a lab, and to the best of my knowledge that question has still not been conclusively answered a year later (I may be unaware of that being decided). My question is: Why do we believe that the Covid-19 virus is (possibly) naturally occurring, given that for the tens of thousands of years of human history, prior to 2019, we have no evidence (to my knowledge) of it ever being encountered, and it is sufficiently different from anything we had seen before that we (humans as a whole) were unequipped to deal with it sufficiently (reducing the probability of it being a recent mutation of something we had seen before)? What sorts of precedents exist for this sort of situation, where a very virulent, potent, and communicable disease has existed in nature for a long period of time, but has never been discovered until "recently" (for some definition of "recently" in context)?

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    – Carey Gregory
    Jan 5 at 16:02

Your premise that this virus is "new" in the way you imagine it is false.

The virus that causes COVID-19 is very similar to other viruses we already know about. Similar enough to one that we even call the virus "SARS-CoV-2", the "2" indicating it's the second such virus in that group, and also similar to another virus named MERS-CoV.

It has even more similarity to other viruses we know circulate in other animals, in particular bats. Coronaviruses come in various categories that cause illness in a variety of species. These include forms that circulate in human populations and cause "common cold" symptoms. These viruses have a long history of recent evolution over the history of human civilization, as well as the distant past in other species. Quoting from Wikipedia:

The most recent common ancestor (MRCA) of all coronaviruses is estimated to have existed as recently as 8000 BCE, although some models place the common ancestor as far back as 55 million years or more, implying long term coevolution with bat and avian species.[73] The most recent common ancestor of the alphacoronavirus line has been placed at about 2400 BCE, of the betacoronavirus line at 3300 BCE, of the gammacoronavirus line at 2800 BCE, and of the deltacoronavirus line at about 3000 BCE.

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